Preliminary Results of Microwave Non-Contact Detection and Depth Determination of Disbonds in Low Permitivity and Low Loss Thick Sandwich Composites

摘要

To ensure manufacturing quality and safe use of thick dielectric composite structures it is essential to utilize a nondestructive testing technique for inspecting their structural integrity. As the thickness of these composite structures increase, most of the nondestructive testing (NDT) techniques become less capable of detecting defects. Microwave signals can penetrate deep inside dielectric materials and interact with their inner structure. They are also sensitive to changes associated with boundary interfaces, which makes them very attractive for disbond detection in composite structures [1,2]. In a thick sandwich composite structure a disbond can occur between any two layers (i.e. in the place of an adhesive fine). The results of an electromagnetic model investigating the potential of a microwave NDT method for detecting disbonds and the potential of determining their depths in a multi-layered sandwich composite is presented. The model describes the interaction of microwave signals, radiating out of an open-ended rectangular waveguide, with a multi-layered composite structure. The composite structure under consideration includes thirteen layers of various materials (three layers of foam with two skin laminates and two similar substrates in between the foam layers) and two layers of air (standoff in front of the sample and free-space backing). Each layer is bound to another by a thin layer of adhesive. A layer representing a disbond is considered to be present in between any given two layers, replacing the adhesive line. The goal of the modeling is to arrive at optimum measurement parameters (frequency and standoff distance) for detecting a disbond and providing information about its depth.